The FBI and U.S. Postal Service agents are investigating bogus official-looking letters sent to voters in at least 28 Florida counties questioning their citizenship and their eligibility to vote, NBC News has learned.

David Couvertier, a spokesman for the FBI in Tampa, said his office opened up an investigation into the possible attempt at voter intimidation on Wednesday after receiving reports that eligible voters throughout the state have received the letters.

"We're taking it as a serious situation," he said. "We're looking at everything from civil rights violations to election fraud -- to everything in between."

Chris Cate, a spokesman for the Florida Secretary of State's Office, told NBC News, "We believe these letters appear to meet the standard of voter intimidation." Between 50 and 100 such letters have been reported to state officials so far, "and those are only the ones we know about. We're encouraging people to come forward."

The fake letters, which first started showing up last Friday, have been sent under the names of real Florida county election supervisors -- with some correct contact information -- informing the voters that the supervisors have received "information" about their citizenship status, "bringing into doubt your eligibility as a registered voter."

The letters also say the voter must fill out a Voter Eligibility Form in the next 15 days -- and failure to do so will result "in the removal of your name from the voter registration rolls and you will no longer be eligible to vote."

"A non-registered voter who casts a vote in the state of Florida may be subject to arrest, imprisonment, and/or other criminal sanctions," the letters state.

Some of the letters have been received by "longtime, staunch voters who have been exercising their right to vote" for years, Couvertier said. While those people are likely to vote anyway, "Our concern is someone who might not be secure and then questions whether they should vote."

It's not clear who sent the letters, which were machine postmarked in Seattle. Couvertier said the FBI in Tampa is working with its Seattle office to track down the perpetrator.

Cate said a "significant majority have gone to Republican voters, but not exclusively. We've got Democrats who received the letters, we've got independents. We're telling everybody to be on the lookout."